1. Field of the Invention
The present invention relates to a band-pass filter, and a branching filter including the band-pass filter.
2. Description of the Related Art
Mobile communication systems conforming to the Long Term Evolution (LTE) standard have become practically used in recent years, and further, practical use of mobile communication systems conforming to the LTE-Advanced standard, which is an evolution of the LTE standard, is under study. Carrier Aggregation (CA) is one of the key technologies of the LTE-Advanced standard. CA uses multiple carriers called component carriers simultaneously to enable wideband transmission.
A mobile communication apparatus operable under CA uses multiple frequency bands simultaneously. Accordingly, such a mobile communication apparatus requires a branching filter capable of separating a plurality of signals in a plurality of frequency bands from each other simultaneously.
A branching filter for separating a first signal of a frequency within a first frequency band and a second signal of a frequency within a second frequency band higher than the first frequency band from each other typically includes a common port, a first signal port, a second signal port, a first filter provided in a first signal path leading from the common port to the first signal port, and a second filter provided in a second signal path leading from the common port to the second signal port. Examples of the first filter include a low-pass filter and a band-pass filter, and examples of the second filter include a high-pass filter and a band-pass filter.
A branching filter that uses a band-pass filter as the first or second filter has an advantage in its capability of increasing attenuation outside the first frequency band in the first signal path and increasing attenuation outside the second frequency band in the second signal path.
Among known band-pass filters are LC filters formed using inductors and capacitors, and acoustic wave filters formed using acoustic wave resonators. The acoustic wave resonators are resonators formed using acoustic wave elements. The acoustic wave elements are elements that use acoustic waves. The acoustic wave elements include surface acoustic wave elements using surface acoustic waves, and bulk acoustic wave elements using bulk acoustic waves.
JP-2010-141859A discloses a branching filter (diplexer) including two band-pass filters each composed of an LC filter.
JP-2015-115866A discloses a branching filter including two band-pass filters each composed of an acoustic wave filter.
Mobile communication apparatuses may require a branching filter for separating two signals in two mutually relatively close frequency bands from each other. Such a branching filter requires a filter having an insertion loss characteristic that abruptly changes in a frequency region close to the cut-off frequency.
It is typically difficult for LC filters to provide an insertion loss characteristic that abruptly changes in a frequency region close to the cut-off frequency.
On the other hand, while acoustic wave filters are suitable to achieve an insertion loss characteristic that abruptly changes in a frequency region close to the cut-off frequency, they are not suitable to provide a wide passband.
Thus, it has conventionally been difficult to provide a branching filter suitable to separate two signals in two mutually relatively close frequency bands from each other, and to provide a band-pass filter suitable for use in such a branching filter.